Three-dimensional viewing apparatuses have been known heretofore mainly for viewing stereoscopic photographs and for projection of stereoscopic images. In the case of devices for viewing of stereoscopic photographs, both images of the pair of stereoscopic images are formed with lenses or mirrors positioned close to the eye. Two different methods are known. In the case of the anaglyphic method, two separate ranges of wavelengths with corresponding chromatic filters in front of the eyes are used. In the case of the Nimslo method, two partial images are nested into one another and are viewed through a pressed-on screen of cylindrical lenses.
For conferences, lectures and movie presentations it is possible to project three-dimensional images. This is done either with special stereo-diaprojectors or stereo overhead projectors as well as with a combination of two flat-picture diaprojectors. The separation of the image is achieved through polarization filters in front of the projector objectives and through polarization spectacles which are worn by the viewer. The mentioned anaglyphic method is also used for stereo projection.
Further, a three-dimensional image viewing apparatus has become known in which an elastic concave mirror is oscillated in such manner that its spherical radius is periodically subject to small changes. Images which are projected in proper phase relation onto the concave mirror, reproducing different sections of a spatial object, can be fused into a single stereoscopic image by the viewer. This apparatus has been suitable only for a depth of about 30 cm of the object to be spatially reproduced.
Considerable efforts have been made in addition to create stereoscopic television. Thus far, however, no apparatus has been developed which can work without optical auxiliary means close to the eye.